The present invention relates in general to optical filters, and in particular to thin film interference filters.
Thin film interference filters are widely used in a variety of optical systems. Such filters are generally implemented in an optical system for reflecting one or more spectral bands of an optical signal, while transmitting others. The reflected or transmitted range, for example, may include wavelengths carrying information sensed or transmitted by the system. Failure or inadequate performance of these filters can thus be fatal to operation of a system in which they are utilized.
Interference filters are wavelength-selective by virtue of the interference effects that take place between incident and reflected waves at boundaries between materials having different indices of refraction. Typically, an interference filter includes multiple layers of two or more dielectric materials having different refractive indices. Each layer is very thin, i.e. having an optical thickness (physical thickness times the refractive index of the layer) on the order of order of xc2xc wavelength of light. The layers may be deposited on one or more substrates, e.g. a glass substrate, in various configurations to provide long-wave-pass (also called long-pass), short-wave-pass (also called short-pass), band-pass, or band-rejection filter characteristics.
Conventionally, the thin film layers in very high spectral performance interference filters for use at wavelengths below about 1200 nm have been implemented using xe2x80x9csoft coatings.xe2x80x9d Soft coatings are typically deposited on a substrate using physical vapor deposition methods such as resistive evaporation and electron-beam evaporation. In these deposition methods, the selected coating material is vaporized, forming a cloud or stream that is imparted to the substrate. Conventional soft coating materials include metals like aluminum (Al) and silver (Ag), and dielectrics like lead fluoride (PbF2), zinc sulfide (ZnS), and cryolite (Na5Al3F14). The vaporized material solidifies on the substrate forming a thin film layer having a density and structure commensurate with the level of energy carried by the vaporized particles.
A major disadvantage associated with soft coatings is that, as the name implies, the coatings are physically soft and susceptible to damage and deterioration in most operating environments. In fact, soft coatings may be easily scratched when contacted by glass, metal, or even plastic. As such, these coatings must be protected from the environment when used in high performance applications, such as fluorescence detection systems, optical communication systems, etc. Also, because they are not very dense, they absorb moisture from the air, which causes their spectral properties to shift and can lead to longer term permanent degradation.
High performance soft coatings are, therefore, usually partially or fully hermetically sealed from the environment by placing them on the inside facing surfaces of two or more pieces of glass in a sealed ring housing, or they are sandwiched between glass substrates cemented together with optical adhesives, thus providing a barrier to moisture. FIG. 1 illustrates an exemplary prior art interference filter structure 100 including soft coating filters 102, 104 sandwiched between glass substrates 106, 108. The illustrated construction is a bandpass filter including a long-wave-pass filter 102 deposited on a first substrate 106 and affixed to the second substrate 108 via an adhesive layer 110. A short-wave-pass filter 104 is deposited on an opposing surface of the second substrate 108 and is affixed to a colored glass layer 112 by an adhesive layer 114. In addition to the effort and expense of hermetically sealing these soft coating filters, the additional substrates and optical adhesives used for such configurations lead to added loss (e.g. due to scattering and absorption) and manufacturing complexity (resulting in increased time and cost to manufacture). For example, in order to minimize deviation of a light beam passing through the filter construction in an imaging application, as in an optical microscope, the overall construction must have a minimal wedge angle; however, when two or more pieces of glass are cemented together, it is difficult to ensure parallelism of the interfaces and hence minimal wedge angle. Another contributor to the manufacturing complexity is that in order to minimize losses associated with the additional surfaces resulting from multiple pieces of glass, additional anti-reflection (AR) coatings must be applied to these surfaces. Because of the increased cost and time required to apply additional coatings, these are often ignored; hence there is a trade-off between manufacturing complexity and filter throughput performance. Furthermore, the excess thickness associated with the hermetic seal makes it impractical for such filters to be diced into very small (e.g., millimeter-sized) filter xe2x80x9cchips.xe2x80x9d
Accordingly, there is a need for a high performance interference filter that is durable, highly reliable, and cost-effective to produce, yet achieves equal or superior optical performance to the current state-of-the-art.
According to one aspect of the invention, there is provided an optical filter including: a substrate; a first thin-film interference filter disposed directly on a first surface of the substrate, and a second thin-film interference filter disposed directly on a second surface of the substrate opposed to the first surface. The first interference filter includes a first plurality of hard coating thin film layers of alternating high and low index of refraction and is configured for transmitting a first range of wavelengths. The second thin-film interference filter includes a second plurality of hard coating thin film layers of alternating high and low index of refraction and is configured for transmitting a second range of wavelengths. The second range of wavelengths is different from the first range of wavelengths, the first and second thin-film interference filters thereby establishing a bandpass transmission characteristic for the filter.
According to another aspect of the invention, there is provided an optical filter including: a substrate; and a first thin-film interference filter disposed directly on a first surface of the substrate, and a second thin-film interference filter disposed on a second surface of the substrate. The first interference filter is configured for transmitting a first range of wavelengths and includes at least 30 hard coating non-quarter wave first filter layers of alternating high and low index of refraction, whereby there is no wavelength in the first range of wavelengths for which the at least 30 hard coating non-quarter wave first filter layers is one-quarter of a wavelength in thickness. The second interference filter is configured for transmitting a second range of wavelengths and includes at least 30 hard coating non-quarter wave second filter layers of alternating high and low index of refraction, whereby there is no wavelength in the second range of wavelengths for which the at least 30 hard coating non-quarter wave second filter layers is one-quarter of a wavelength in thickness.
According to yet another aspect of the invention there is provided a fluorescence spectroscopy system including: a source of light; and an excitation filter for selecting an excitation band of wavelengths from the light to be directed onto a sample under test. The light may be coherent, e.g., laser light, or incoherent light. Another fluorescence spectroscopy system consistent with the invention includes: a source of light for illuminating a sample under test with an excitation band of wavelengths; and an emission filter for selecting an emission band of wavelengths from a fluorescence signal transmitted by the sample under test in response to illumination by the excitation band of wavelengths. The excitation and/or emission filters may include a substrate, a first thin-film interference filter disposed directly on a first surface of the substrate, and a second thin-film interference filter deposited directly on a second surface of the substrate opposed to the first surface. The first thin-film interference filter includes a first plurality of hard coating thin film layers of alternating high and low index of refraction and is configured for transmitting a first range of wavelengths. The second interference filter includes a second plurality of hard coating thin film layers of alternating high and low index of refraction and is configured for transmitting a second range of wavelengths different from the first range of wavelengths. The first and second thin-film interference filters thereby establish a bandpass characteristic for the optical filter for transmitting the band of wavelengths.
A method of selecting a band of wavelengths from light in a fluorescence spectroscopy system consistent with the invention includes: providing an optical filter and imparting the light on the optical filter. The optical filter includes a substrate, a first thin-film interference filter disposed directly on a first surface of the substrate, and a second thin-film interference filter deposited directly on a second surface of the substrate opposed to the first surface. The first thin-film interference filter includes a first plurality of hard coating thin film layers of alternating high and low index of refraction and is configured for transmitting a first range of wavelengths. The second interference filter includes a second plurality of hard coating thin film layers of alternating high and low index of refraction and is configured for transmitting a second range of wavelengths different from the first range of wavelengths. The first and second thin-film interference filters thereby establish a bandpass characteristic for the optical filter for transmitting the band of wavelengths.